Conventionally, unwoven fabrics have been formed by transporting fiber material after the fiber material is supplied on an endless running mesh belt. Many kinds of methods of forming the unwoven fabric have been known, and new technology for forming the unwoven fabric has been also developed.
It is difficult to clearly classify the methods of forming the unwoven fabric. When they are classified mainly in accordance with the process for binding the fibers, one is a thermal bonding method in which compound fibers of the core and sheath type with which a resin with a low melting point is covered are adopted, or the fiber material with which a resin with a low melting point powder is mixed is supplied on the mesh belt, so that the low melting resin is melted by the heating or an ultra-sound welder to cause fusion between the fibers, whereby the nonwoven fabric is formed. In addition, one is a resin bonding method in which the fiber material is supplied on the mesh belt where an adhesive resin is impregnated and then dried to form the nonwoven fabric. Further, there are the chemical bonding method and the span racing method in which the fibers are intersected by high-pressure water flow.
When the method of forming nonwoven fabric is classified in accordance with the process of supplying the fiber material, the carding method in which the fiber material is supplied using the carding machine, the air raid method in which the fiber material which has been unwoven is supplied using air, or the span bonding method in which yarn which is spun from fiber material in a form of yarn is directly supplied on the mesh belt without using the fibers which has been formed into the nonwoven fabric, and the adjacent fibers are fused by the heating, etc. In addition, the melt blow method in which the fibers are spun in the form of mist to be supplied on the mesh belt is known.
As stated above, there are various kinds of methods of forming the nonwoven fabric. In particular, in the span bonding method, good lateral rigidity, good air permeability, and good sheet supportability are required for the nonwoven fabric. More specifically, since the nonwoven fabric is caused to shift laterally during its running, it is necessary to apply a palm to the nonwoven fabric to rectify its shift. That is why the nonwoven fabric can fold due to its contact with the palm, if the lateral rigidity of the nonwoven fabric is low.
In addition, it is necessary to set the air permeability required for the nonwoven fabric appropriately in accordance with the nonwoven fabric to be formed. More specifically, if the air permeability is too high, the fibers can be removed, while, on the other hand, if it is too low, an effect of vacuuming can be reduced. Furthermore, when the nonwoven fabric is transported, the nonwoven fabric can be folded due to the fact that the formed nonwoven fabric can shift on the fibers if the sheet supportability of the nonwoven fabric is too low.
More specifically, a conventional fabric is shown by FIG. 19 in Patent document 1 (Japanese Patent No. 2558154). In such a conventional fabric, the fibers can be stuck into the fabric as the time elapses due to its repeated use in the forming process of the nonwoven fabric, although it has initially very good air permeability. Here, the fibers sticking phenomenon is defined to be the one in which the fibers can enter into a space between intersections of knuckles of wires. If such a phenomenon occurs, the wires can dig into the nonwoven fabric, or the air permeability of the fabric can be reduced.
FIG. 15 is a picture which shows a situation in which the fibers of the conventional fabric are stuck. As readily seen from FIG. 15, the fibers enter into a space where yarns are woven. This fiber sticking situation can be generated because the force by which the intersection of the wires is supported is low. In other words, if such a supporting force is low, the wire can rattle during its transportation, so that the fibers can be sandwiched between the space between intersections of knuckles of wires.
Here, the force by which the intersection of the wires is supported is defined to be the one which is applied to both of a weft and a warp at a knuckle portion. Generally, the supporting force is high when the knuckle is constituted by a single yarn, whereas, the supporting force tends to be low when the knuckle is constituted by multiple yarns.
That is why the weave design in which the supporting force is the highest is a plain weave design. Because, in the plain weave design, each of all the knuckles is constituted by a single yarn, so that the density of the knuckles becomes the highest.
However, in a conventional fabric, such a plain weave design cannot be adopted because the surface density can be reduced if the diameter of the weft is increased, which causes an incompatibility between the lateral rigidity and the surface density. That is why the plain weave design has not been adopted in the conventional fabric.    Patent Publication 1: Japanese Patent No. 2558154